In recent years, from a viewpoint of preventing global warming, a technology which utilizes hydrogen as a medium for transporting or storing energy has been developed in order to reduce the discharging of greenhouse gases (CO2, NOx, and SOx). Thus, development of a metal material used for devices for storing and transporting hydrogen is expected.
In the related art, a cylinder made of thick (thickness is large) Cr—Mo steel is filled or stored with a hydrogen gas having a pressure of about 40 MPa as a high pressure gas. In addition, a SUS316 type austenitic stainless steel (hereinafter, referred to as “SUS316 steel”) of the Japanese Industrial Standards is used as a piping material or a high pressure hydrogen gas tank liner of a fuel-cell vehicle. The hydrogen embrittlement resistance characteristics of the SUS316 steel in a high pressure hydrogen gas environment is more satisfactory than, for example, a carbon steel including the aforementioned Cr—Mo steel or SUS304 type austenitic stainless steel (hereinafter, referred to as “SUS304 steel”) of the Japanese Industrial Standards.
In recent years, prior to general sales of fuel-cell vehicles, an official trial production or demonstration experiment of a hydrogen station has been in progress. For example, a hydrogen station, in which a large amount of hydrogen can be stored as liquid hydrogen and the pressure of the liquid hydrogen is increased to supply a high pressure hydrogen gas having a pressure of 70 MPa or greater, is in the demonstration (validation) phase. In addition, in the hydrogen station, a technology, which is referred to as precooling, has been practically used, and in the technology, hydrogen which is to be filled in a tank of the fuel-cell vehicle is pre-cooled to a low temperature of about −40° C. From the above-circumstances, it is conceived that a metal material used for a storage container for liquid hydrogen attached to a dispenser of the hydrogen station or hydrogen gas piping is exposed to a hydrogen gas having a high pressure of 70 MPa and a low temperature.
As a metal material in which hydrogen embrittlement does not occur in a severe hydrogen embrittlement environment, the SUS316 steel and SUS316L steel containing about 13% of Ni are exemplary examples. Use of these two types of steels in a 70 MPa-class hydrogen station in Japan is permitted by the standards determined by the High Pressure Gas Safety Institute of Japan.
Meanwhile, in order to construct and autonomously develop a hydrogen energy society where a fuel-cell vehicle is mainly used in the future, it is essential to reduce the cost of fuel-cell vehicles or hydrogen stations. That is, in order to reduce the use amount of the steel material caused by the reduction in size and thickness of various devices, the strength of the metal material used in a hydrogen embrittlement environment is required to be further increased.
However, the SUS316 type austenitic stainless steel described in the aforementioned exemplified standard is expensive since the SUS316 type austenitic stainless steel includes a large amount of Ni and Mo, which are rare metals. Furthermore, a tensile strength of about 650 MPa is required to be used for the purpose of high pressure hydrogen piping. However, even in the case where the SUS316 type austenitic stainless steel is subjected to a solutionizing treatment, the SUS316 type austenitic stainless steel does not satisfy the above tensile strength. Thus, the SUS316 type austenitic stainless steel is subjected to cold working to reinforce the strength and is then used.
Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2002-371339) discloses a stainless steel including 5% to 9% of Ni, which is low, and having a low cost.
In a stainless steel disclosed in Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2002-173742), the metallographic structure (metal structure, microstructure) is controlled to have a dual phase structure of an austenite phase and a martensite phase by a thermomechanical treatment, while the amount of Ni is set to 4% to 12%. Thereby, a remarkably hard stainless steel is achieved which has a Vickers hardness of about 500.
The stainless steel disclosed in Patent Document 3 (PCT International Publication No. WO 2004/83477) is a stainless steel for a high pressure hydrogen gas, which is aiming for increasing the strength by solid solution strengthening of N. This stainless steel has the strength higher than the strength of SUS316 steel, while satisfactory hydrogen embrittlement resistance characteristics are secured.
In the stainless steel disclosed in Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2009-133001), hydrogen embrittlement resistance characteristics are enhanced by utilizing carbonitrides of Ti and Nb having sizes of 1 μm or greater, and the stainless steel is economically excellent since addition of Mo to the SUS 316 steel is omitted.
However, the stainless steel disclosed in Patent Document 1 has almost the same strength as that of the SUS316 steel, and the use of the stainless steel in a hydrogen environment is not considered.
In addition, since the stainless steel disclosed in Patent Document 2 includes a martensite phase in which hydrogen embrittlement easily occurs, it is difficult to apply this stainless steel in a hydrogen environment.
In addition, the stainless steel disclosed in Patent Document 3 substantially includes Ni at an amount of 10% or more, and in the case where the amount of Ni is reduced to less than the above-described amount, it is required to add Mo, Nb, V, or Nd; and as a result, the cost becomes high.
In addition, the stainless steel disclosed in Patent Document 4 has almost the same strength as that of SUS316 steel, and enhancement of the strength is further desired.
As such, currently, a high-strength austenitic stainless steel has not appeared yet, which has both economic properties and hydrogen embrittlement resistance characteristics in a low temperature and a high pressure hydrogen gas environment exceeding 40 MPa.